Vehicles with engines require engine exhaust aftertreatment systems to remove unwanted chemicals from the exhaust flow and meet emissions requirements. The engine exhaust aftertreatment system may be a catalytic converter. In the case of a three-way catalytic converter, various amounts of carbon monoxide, unburned hydrocarbons and nitrous oxides are removed from the engine exhaust flow before it exits the vehicle to the environment. Unburned hydrocarbons may include unburned fuel and partially burned fuel. If high levels of unburned hydrocarbons are permitted to reach the catalytic converter, the catalytic converter temperature may increase due to chemical reactions between the unburned hydrocarbons and oxygen caused by the catalyst material. These chemical reactions release heat. The temperature rise in the catalytic converter may lead to catalyst degradation or damage, with the potential for catalyst meltdown, restricted exhaust flow, and catalyst deactivation.
Hybrid vehicles may have electric motors configured to rotate the engine without combustion occurring. This hybrid architecture may allow high levels of unburned hydrocarbons reaching the catalytic converter if the electric motor is rotating the engine while the engine is misfiring, stalling, or complete combustion is otherwise not occurring within a cylinder.
A system and method needs to be provided to monitor the engine and exhaust to detect and/or prevent large amounts of unburned hydrocarbons from reaching the catalytic converter, or to provide a diagnostic code when it does occur.